The present invention relates to temperature effects on laser instruments, and in particular, to a lens whose change in refractive index with temperature and wavelength compensates for temperature effects on both the wavelength of light produced by a laser source and the length of the mounting for the lens and source.
Laser instruments have found wide application in many industries including construction and agriculture, where laser instruments are subjected to uncontrolled and often harsh temperature conditions. Some such instruments, for example automatic levelling instruments, include transmitters which are designed to project a laser beam over large distances and to establish a plane of light for reception or reflection by a target. The transmitter typically includes a laser source, such as a laser diode or He-Ne laser tube, a lens which produces a substantially collimated laser beam, and a rotating mirror or other device to establish a plane of laser light. In many cases, the lens used with a laser diode is a triplet lens, comprised of three lens elements separated by air spaces. The laser diode source and lens are typically assembled on or in a mounting structure, such as a tube or "barrel", where they are separated by a fixed, design distance equal to the focal length. The laser diode source is thus positioned at the focus of the lens to produce a laser beam having desired beam characteristics such as collimation or a minimum focused spot size at design distances.
In some construction and agricultural applications laser instruments may be subjected to wide variations in temperature from ambient, i.e. approximately 20.degree. Centigrade (.degree.C.). Changes in temperature, T, cause a number of changes in the laser transmitter, however. The wavelength of light emitted by a laser source, particularly a laser diode, will shift, causing a change in the index of refraction, n, of the lens corresponding to its Abbe V-number, and the index of refraction of lens materials will change due to its .DELTA.n/.DELTA.T characteristic, both of which defocus the lens. Further, the mounting expands or contracts, changing the separation between the laser source and the lens, further defocusing the transmitter. In turn, these changes adversely effect the desired beam characteristics, such as collimation or minimum focused spot size at design distances. Such changes are more pronounced in transmitters which use laser diodes, as the separation between a laser diode source and lens is small, and the effect of temperature on the wavelength of light produced by the diode is significant.
In some existing laser transmitters these temperature effects have simply been endured without remedy. In others, multiple material mounting structures have been developed which attempt to expand and contract the mounting to match the changes in the focal length of the lens. However, no single mounting material has been found to match such changes, and the mounting structures have become both more complex and expensive than is desirable.
Accordingly, less expensive and less complex solutions are sought to the problems induced by temperature effects in laser instruments, particularly where laser diode sources are used.